1. Field of the Invention
This invention relates to an optical data medium driving apparatus, such as an optical disc or a photomagnetic disc, and more particularly to an optical data medium driving apparatus designed by paying attention to an amplifier for an output current of a detector detecting an error signal for driving a tracking servo mechanism and a focusing servo mechanism, and relates to a servo circuit for position controlling the light spot of, for example, an optical disc apparatus, especially a servo circuit inexpensive to be produced and of high reliability.
2. Description of the Prior Art
Recently, an information processing amount by a computer has increased steadily and an optical disc apparatus of larger recording amount has been noticed in order to record and reproduce the increased information, therefore the technology thereof is remarkably improving. In the optical disc apparatus, the information recorded on a data medium or to be recorded thereon is a micropit of 1 .mu.m or less, whereby a focusing servo mechanism or a tracking servo mechanism is indispensable which restricts the laser beam to a micro spot light of diameter of about 1 .mu.m to allow the laser light to impinge always on the medium surface regardless of surface deflection or track deflection of the data medium, whereby development of a servo mechanism of high accuracy has been desired.
In the conventional optical disc apparatus or a photomagnetic disc apparatus, when the information is recorded, there is a difference in the relative peripheral speed between an optical head and a disc at an inner periphery or an outer periphery. Hence, for obtaining the equivalent condition on the disc it is required to intensify the recording laser beam as it approaches the outer periphery. For this control, the information of track number prerecorded on the disc is demodulated and D/A converted, thereby controlling the laser power.
Meanwhile, since the servo system for driving the optical head, such as a tracking servo or a focusing servo, utilizes the reflected or transmitted light, a servo driving signal changes when the laser power changes or a reflection factor changes. Accordingly, in order to obtain a stable servo driving signal, the servo gain is required to be automatically changed so that the averaged servo driving signal may be obtained regardless of variation in the laser power and reflection factor of the disc.
Next, explanation will be given on a concrete example of an amplifier which automatically changes the servo gain, disclosed in the Japanese Patent Application Laid-Open Gazette No. 22746/1985.
FIG. 1 shows the conventional servo gain control circuit, in which reference numeral 117 designates a photosensor including four receiving surfaces known in the cylindrical lens method, which outputs from the upper side shown in the drawing three kinds of systems of a track error signal, a focus error signal and a total quantity of reflected light signa. Reference numeral 118 in the track error signal system designates a differential amplifier, and 119 and 121 designate amplifiers respectively. These amplifiers 118, 119 and 121 are connected in series and output a track servo driving signal to an output terminal 122. Reference numeral 123 in the focus error signal system designates a differential amplifier, and 124 and 126 designate amplifiers respectively, these amplifiers 123, 124 and 126 being connected in series and outputting the focus servo driving signal to an output terminal 127. The total quantity of reflected light system is so constituted that an inverting amplifier 128 and a filter 129 are connected in series, an output of the filter 129 is of negative polarity and connected in parallel to gate terminals G of field-effect transistors (to be hereinafter called FET) 120 and 125, the source terminals S of the FETs 120 and 125 are grounded, and the drain terminals D are connected to one input terminal of amplifiers 119 and 124 respectively.
FIG. 2 is a graph showing an example of a characteristic curve of gate voltage V.sub.GS to a resistance value R.sub.DS between the equivalent drain and the source of the FETs, which has a characteristic of a curve of secondary degree, wherein the center point of a linear region is represented by P.
Next, explanation will be given on operation of the servo gain control circuit shown in FIG. 1.
The output of the amplifier 128 is a total quantity of reflected light signal, which is removed by the filter 129 of noise component and then connected in parallel to gate terminals G at the FETs 120 and 125 respectively, so that the level variation in the total quantity of reflected light signal is applied as gate voltage V.sub.GS to the terminals G, and a resistance value R.sub.DS between the equivalent drain and source corresponding to the gate voltage V.sub.GS represented by the characteristic curve shown in FIG. 2 varies. When a resistance value of FET 120 is represented by R.sub.DS4, that of FET 125 by R.sub.DS9, resistance between one input terminal and the output terminal at the amplifier 119 by R.sub.3, a resistance between the same at the amplifier 124 by R.sub.8, and amplifier gains of the amplifiers 119 and 124 by A.sub.3 and A.sub.8, A.sub.3 and A.sub.8 are given in the following expressions: ##EQU1## whereby the amplifier gains are automatically controlled corresponding to variation in the resistance value R.sub.DS of each FET. In other words, as the total quantity of reflected light reduces, gate voltage V.sub.GS decreased in proportion thereto. Conversely, the servo driving signal output is designed to be averaged by utilizing an increase in the amplifier gain.
Since an intensity ratio of the laser light is large during the recording and the reproducing of the information, when the information is reproduced, the output of total quantity of reflected light, that is, the output of amplifier 128, is small, whereby a region of bad linearity at the FET characteristic shown in FIG. 2 is obliged to be used, thereby causing the defect that sufficient amplifier gain is not obtainable and a stable servo circuit is not realizable.
As the countermeasure for the above conventional defect, it is considered that during the reproduction the servo gain is made larger and during the recording the servo gain is changed corresponding to the output of total quantity of reflected light of an optical head, so that an automatic adjusting circuit for servo gain obtainable of a stable drive output for the optical head both when the information is reproduced and recorded regardless of variation in the total quantity of reflected light, is used.
In the disc apparatus provided with amplifiers for amplifying the track error signal and focus error signal from the optical head so as to adjust the servo gain for operating the tracking servo and focusing servo by the outputs of respective amplifiers, it is considered as the improvement for the above defect that a changeover switch for switching the gains of both the amplifiers by the switching signal for the recording and reproducing is provided and control means which stabilizes the outputs of both the amplifiers at about the equal level in spite of separation of the recording from the reproducing is provided.
FIG. 3 is a structural view of an example of the automatic adjusting circuit for the servo gain by applying the above improvement, in which only the components different from the conventional example shown in FIG. 1 are shown.
In FIG. 3, reference numerals 130 and 131 designate analog switches having contacts 136, 137 and 138, 139 respectively, which are driven by recording/reproducing gate signals inputted to the terminal 133, 132 designates an amplifier into which the total quantity of reflected light signal passing a filter 129 is inputted, and is output connects to the gate terminal G of a FET 120 through a contact 138 of the analog switch 131. The output of the filter 129 is branched to connect to the gate terminal G of the FET 120 through a contact 139. The source terminal S of the FET 120 is grounded, and the drain terminal D connects with one input terminal of an amplifier 119 and connects in common with the output terminal of the amplifier 119 through contacts 136 and 137 and resistances R134 and R135 at the analog switch 130 respectively.
Next, an explanation will be given on operation of the circuit shown in FIG. 3. The contacts 136 and 138 at the respective analog switches 130 and 131 are closed and those 137 and 136 are open when the input signal to the terminal 13 is reproduced, and they operate reversely when the input signal is recorded.
At first, the total quantity of reflected light signal obtained by all the composite outputs of the photosensor 117 shown in FIG. 1 is amplified so that the output is of negative polarity by the inverting amplifier 128. When the output is inputted as gate voltage to the FET 120 through the filter 129 at the next stage, the gain of amplifier 128 is so set that the mean value of inputted gate voltage, when the information is recorded, is positioned to get a proper linearity in FIG. 2 (namely the point P). When the information is reproduced, the laser power is lowered, and the amplifier (132) gain is so set that the inputted gate voltage at that time is about equal to the mean value during the recording. At this state, when the recording/reproducing gate signal is applied to the terminal 133, the input gate voltage of the FET 120 during the recording vertically varies in a range of nearly proper linearity around the point P on the characteristic curve in FIG. 2 and the gate voltage during the reproduction does not so fluctuate from the output (gate voltage access to the operating point P) decided by the function limit of amplifier 132.
Next, the servo gain is set. For example, when the track error signal of photosensor 117 is amplified by the differential amplifier 118, its output represents the direction and extent of track error, thereby being connected to a coil moving an objective lens to correct the track error through the amplifiers 119 and 121. Herein, the servo gain is set by the amplifiers 118 and 119, but the amplifier(119) gain is decided by the resistance value R.sub.DS4 between the equivalent drain and source of FET 120 and the resistances R134 and R135. When the amplifier (119) gain is represented by A.sub.3W when recorded and by A.sub.3R when reproduced, the gain is given in the following expressions: ##EQU2##
Here, when the gain of amplifier 128 is set on a basis of the mean value of the laser power during the recording, since the laser power during the reproducing is low, even when the gain of amplifier 132 is set, the servo gain tends to be short. Hence, the analog switch 130 is controlled by the recording/reproducing gate and values of resistance R134 and R135 are selected so as to obtain driving voltage to correct the track errors to about an equal extent during the recording and reproducing, thereby enabling the most suitable servo gain to be set.
Now, the circuit shown in FIG. 3 is described as to track servo driving signal system, the focus error signal system is applicable in the same way as in FIG. 3. In the FIG. 3 circuit, the differential amplifier 118 is replaced by that 123 and the amplifiers 119 and 121 by those 124 and 126, thereby enabling a stable focus servo driving signal to be obtained.
As above-mentioned, the improved automatic gain control circuit for the servo gain is simple in construction, and, even when the output of the total quantity of reflected light fluctuates regardless division of the information for recording/reproducing, can always easily obtain the stable servo driving signal.
The conventional optical data medium driving apparatus and an improvement thereof are constructed as above-mentioned. In other words, the apparatus is provided with function (to be hereinafter called the auto gain control: AGC) such that the servo driving signal is divided by the total quantity of reflected light to average the servo driving signal to thereby maintain constant the servo loop gain regardless of variation in the laser power and reflection factor of the disc. Also, in order to compensate a narrow dynamic range of AGC, the gain switching stage for switching the gain at the step of reproducing/recording is provided at the preceding stage of the AGC.
In consideration of accuracy (about .+-.1 .mu.m for focusing servo and about .+-.1 .mu.m for tracking servo) and band (about 3 KHz for the gain crossover frequency) required to the optical disc servo, as the characteristic of AGC, the dynamic range is limited to five times through ten times the extent.
However, there are some data media for the optical disc having various characteristics. In consideration of the reflection factor after recording inclusive, variation in the reflection factor of the optical disc becomes about ten times larger.
The reproduction power is required to be changed to meet with the sensitivity of data medium and not to break the recording data. Also, the recording power is different depending on the kind of data medium. Furthermore, in consideration of an apparatus, such as the photomagnetic disc, which continuously lights the semiconductor laser by the power equivalent to or larger than the recording power to thereby erasing the recording data, even average variation in the laser power becomes about ten times larger.
Accordingly, for the optical disc or the photomagnetic disc, as a recording medium, having various characteristics mentioned above, the problem is created in that the conventional example only enlarging the AGC dynamic range by switching the preceding reproduction/recording is insufficient.
Meanwhile, there is another prior art relating to the AGC.
FIG. 4 is a block diagram of the servo circuit of the conventional optical data medium apparatus disclosed in the Japanese Patent Publication Gazette No. 28653/1983.
In FIG. 4, reference numeral 201 designates a photosensor for detecting a focus error of the light spot (not shown), which comprises two-divided light receiving surfaces 201a and 201b, reference numerals 202 and 203 designate current-voltage converters (to be hereinafter referred to as IV converter) which convert photocurrent signals inputted from the respective light receiving surfaces 201a and 201b into voltage. Reference numerals 204 and 205 designate operational amplifiers for obtaining a difference signal V.sub.X and a sum signal V.sub.Y of each output of the respective IV converters 202 and 203. Reference numeral 206 designates an analog divider which divides the difference signal V.sub.X by the sum signal V.sub.Y and constitutes normalization processing means for normalizing by the sum signal V.sub.Y the level variation of difference signal V.sub.X to be constant. Reference numeral 207 designates an actuator driving circuit for amplifying the output of analog divider 206. Reference numberal 208 designates a focus actuator for controlling the focal position of the light spot. The above components 201 through 208 comprise the auto focusing servo system.
Similarly, the auto tracking servo system comprises a photo sensor 209 comprising the two-divided light receiving surfaces 209a and 209b for detecting the track error of the light spot, IV converter 210 and 211, an operational amplifier for obtaining the difference signal V.sub.Z of the photosensor 209, analog divider 213, actuator driving circuit 214, and tracking actuator 215 for controlling tracking of the light spot. In addition, the sum signal V.sub.Y inputted to the analog divider 213 at the auto-tracking servo system uses the sum signal V.sub.Y of the division amplifier 205 at the auto focusing servo system.
Next, explanation will be given on operation of the conventional servo circuit shown in FIG. 4.
At first, the light receiving surfaces 201a and 201b at the focus error sensor 201 output a photocurrent corresponding to a received light quantity, the IV converters 202 and 203 converting current signals from the light receiving surfaces 201a and 201b into voltage.
The operational amplifiers 204 and 205 compute the difference signal V.sub.X and sum signal V.sub.Y of voltages from the respective IV converters 202 and 203, the analog divider 206 divides the difference signal V.sub.X by the sum signal V.sub.Y. The actuator driving circuit 207 amplifiers the output of the analog divider 206 to drive the focus actuator 208 to position-control the light spot to be constant.
Similarly, the actuator driving circuit 214 drives the tracking actuator 215 corresponding to imbalance in quantity of received light on the light receiving surfaces 209a and 209b at the photosensor 209 so that the irradiation position of the light spot is position-controlled on a desired track.
Generally, when variation in the output from the light source for the light spot changes in phase the quantity of received light of the photosensors 201 and 209, the output level of difference signal V.sub.X with respect to the equal control error amount varies. The aforesaid analog divider 206 is used to normalize variation in output level corresponding to the quantity of received light, whereby the quantity of control error of the stable servo loop is obtained.
Such normalization processing means for dividing the difference signal V.sub.X by the sum signal V.sub.Y is well-known as disclosed in, for example, the Japanese Patent Publication Gazette No. 5293/1980.
The conventional servo circuit, as above-mentioned, uses the analog divider 206 in order to normalize the difference signal V.sub.X. Since the analog divider 206 is required to have the frequency characteristic to the extend that a phase lag at the target servo region is not problematical and the dynamic range and accuracy in the range that an offset to cause a control error in the servo loop is not problematical, there is the problem that an expensive IC applied with laser trimming may inevitably be used.
Since the signal normalized by the analog divider 206 is an analog signal, the drift or aging cannot be neglected and the reliability is difficult to maintain thereby creating the problem in that all circuits must be analog processed and high analog technology are required to add function to have high performance.